Many laser systems currently employ a number of optical elements especially in folded cavities used to shorten the cavity to a usable small size. It is important in such systems that the emitted laser beam be aligned with the optical axis of the system called the boresight axis. For many applications such as laser target designators and missile counter measures, alignment accuracy is critical and angular errors arc to be kept below 250 microradians to assure that laser beams hit their targets. While such systems are aligned at room temperature, thermal excursions distort the associated optical benches and their components, and make it difficult to limit angular error. Thus when a beam is re-directed or passed through optical elements, it is important that thermal expansion or contraction does not cause an angular misalignment of the beam with the boresight axis of the equipment.
Laser systems such as ring lasers exist in which a laser beam is passed through a number of optical elements that support for instance Q-switching, amplification, output coupling, and polarization control. Other elements affect angular beam alignment, power monitoring, and beam diversion. Moreover, some of these devices are active devices in that they operate as optical parametric amplifiers or frequency doubling devices. Various elements of the optical system for re-directing the laser beams are intended to reduce sensitivity of the optical system to tilting motions of the optics and bending or twisting of the structure. These include for instance corner cubes, Porro prisms, and Dove prisms.
While it is the purpose of such devices to return a beam along an axis exactly parallel to the incident optical beam, no matter how the optic is tilted during operation, there is a little understood factor which causes subtle but large alignment problems relative to the above-stated boresight requirement. Thermal expansion of the optical bench can cause these devices to laterally shift their exit beams. Furthermore, these optics actually magnify the lateral shift. When these laterally-shifted beams are then focused, the result is a beam which is angularly displaced from the centerline of the focusing optics. Thus while the radiation exiting from a corner cube is returned parallel to the incident radiation, which would seem to be just exactly what one would want, it has been found that the laterally-shifted beam when focused causes the collimated beam to come out angularly shifted from the optical centerline of the lens.
What this means is that if a collimated beam impinges on a retroreflective device at a point on the surface of the device not on the optical centerline of the device due to thermally-induced lateral shifting, then while the returned beam will come out parallel to the incident beam, it's position in space will be laterally-shifted by an amount calculatable in terms of the thermal coefficient of expansion of the materials utilized.
When these components are followed by a lens, as they often are, or when the beams from these devices are introduced into a laser cavity having curved mirrors defining the cavity, then the effect of the lateral shift of the collimated beam is amplified. This is because the beam, instead of impinging directly on the center of a downstream lens, is off-set from the center of the lens. While the collimated beam is in fact still focused at the focal point of the lens, the direction of the beam that comes through the focal point is at an angle to the optical axis of the lens itself. This manifests itself as angular boresight error.
What this means is that the angular direction of an exiting beam is angularly shifted due to a lateral shift of the collimated beam ahead of the lens. When such optical systems are utilized, for instance, as laser target designators or target illuminators, then an angular shift of 250 microradians can cause target illumination or jamming beams to completely miss the intended target.
While the optical system may be aligned at room temperature, when temperature shifts occur one needs to be able to realign the exiting beam with the boresight axis and to do so automatically.
In the past, it has not been understood that the lateral shift of an optical retroreflective element due to thermal expansion causes such misalignment problems. With the realization that a lateral shift ahead of a lens results in angular error, it is incumbent upon the designer of the optical system to be able to provide a shift in the position of the lens so that when a thermal change occurs, the collimated light impinging on the lens is made to come in on the optical centerline of the lens, i.e. along its optical axis.
A failure to understand that a lateral shift in the collimated light impinging on a lens can result in an angular off-set of the exiting beam has led to frustration in the design of optical benches and optical systems which seem not to maintain initial alignment.
Considerable work has been focused on the coefficient of thermal expansion of the various components in the optical system, along with that of the optical bench, so that in theory as the optical bench expands with a thermal rise, there will be a theoretic null result and alignment will be maintained.
Work has also been directed towards providing optical bench materials and components having CTE's, coefficients of thermal expansions, close to one. However, these materials are excessively expensive, sometimes rust and in general are undesirable due to the inability to procure the materials themselves.
The result is that optical benches are made from aluminum, having a CTE for instance of 24 ppm/° C. It is then incumbent upon the designer of the optical system to be able to null out the effects of thermal expansion so that the angular error introduced by thermal expansion is minimized if not completely eliminated.
It will be appreciated, for instance, that an alignment error of as little as 250 microradians due to thermal expansion of the optical bench at one kilometer can result in the laser beam being off by more than the diameter of the beam itself. This problem obviously increases with an increase in the distance of the laser from the intended target.
The problem is even more complicated when, for instance, one laser pumps an optical parametric amplifier to produce multi-color radiation. When lateral off-setting occurs the different colored beams can come out at different angles.